1生长素 Auxin

生长素（auxin）是一类含有一个不饱和芳香族环和一个乙酸侧链的内源激素，英文简称为IAA，其化学本质是吲哚乙酸。另外，4-氯-IAA、5-羟-IAA、萘乙酸（NAA）、吲哚丁酸等为类生长素。生长素是第一个被发现的植物激素。生长素中最重要的化学物质为3-吲哚乙酸。

生长素 Auxin

• Discovery of Auxin
  • 1890s
    • Darwin studied phototropism –movement towards light
  • 1913
    • Boysen-Jensen showed that the transmitted influence can move through a gelatin block
  • 1930s
    • Auxin was purified and shown to promote growth、
    • Auxin’s root-promoting properties were also known.
    • Different tissues were recognized to have different sensitivities to auxin
    • Polar transport of auxin was recognized.
• Types and Structure of Auxin
  • Naturally occuring
    • IAA
    • 4-CI-IAA
    • IBA
  • Synthetic Auxins
    • NAA
    • Dicamba
• Biosynthesis
  • IPA pathway-吲哚-3-丙酮酸途径
    • The IPA pathway is a conserved two-step pathway. The activity level of the two enzymes are regulated in diverse ways
• Metabolism of Auxin
  • Auxin homeostasis is also regulated by conjugation and degradation
    • GH3(IAA氨基化合成酶）
      • Overexpression of an auxin conjugating enzyme encoded by a GH3 gene reduces auxin levels in the plant and causes a dwarfed phenotype.
    • DAO（双加氧酶）
• Polar Auxin Transport
  • Feature
    • Auxin moves long distances through the phloem.
    • Auxin also moves via auxin transport proteins.
    • Auxin normally moves from the tip of the shoot towards the tip of the root. At the root tip, auxin changes direction and moves short distances up the root again (basipetally).
    • Coleoptiles or shoots move auxin to the shaded side
    • Gravitropism is a response to a change in orientation relative to gravity
    • Bending is caused by differential growth mediated by different auxin accumulation
    • Auxin accumulates on the lower side of roots, but they bend down
  • chemiosmotic model （化学渗透模型）
    • 1. IAA enters cell either passively (IAAH) or actively (IAA-)
    • 2. Cell wall is maintained at acidic pH by H+-ATPase
    • 3. IAA predominates in cytosol
    • 4. IAAanions exit cell via auxin anion efflux carriers (PIN proteins)concentrated at the basal end of cell.
  • Auxin has two methods of entering the cell
    • Passive diffusion
    • Cooperative transport driven by the proton dynamic potential – the inwardcotransporter AUX
      • The AUX1/LAX influx carriers contribute to movement of IAA into the cytoplasm.
  • Transporter
    • AUX1 takes up auxin into the cell
    • Auxin transport is facilitated by AUX1/LAX proteins
    • The ABCB proteins are important for auxin transport
    • PIN
      • PIN auxin efflux carriers are encoded differentially expressed genes
      • PIN is a plasma membrane protein with transmembrane segments and a large hydrophilic loop between transmembrane segments five and six
      • PIN proteins orient asymmetrically in plant cells
      • Different PIN proteins orient differently
        • PIN2 localizes to the lower surface of root cortex cells and the upper surface of epidermal cells
      • PIN3 changes polarity in response to a change in the gravity vector
      • The distribution of PIN proteins contributes to the auxin gradients
      • PIN protein distributions contribute to auxinmediated patterns
• Perception and signaling
  • SCF
    • The auxin-receptor TIR1 is a component of the SCF ubiquitin ligase complex SCFTIR1
  • Auxin acts like a molecular glue that holds coreceptor proteins together
  • The canonical auxin signaling pathway
  • Aux/IAA proteins repress the activity of Auxin Response Factors (ARFs)
  • Model for auxin-regulated transcriptional control
    • Low auxin level
      • Some ARFs are complexed with Aux/IAA repressors, interfering with their action
• Physiological Function
  • Auxin-induced cell elongation also contributes to growth control
    • During embryonic development, IAA is dependent from the establishment of zygotic polarity, the formation of the raderogen and root apical meristems, to the formation of stem apical meristems
    • In the model for auxininduced cell wall elongation, auxin stimulates a plasmamembrane bound proton pump, which acidifies the cell wall
    • Wall acidification activates wall loosening enzymes called expansins
  • During Arabidopsis embryogenesis, an auxin gradient forms that is necessary for root formation
    • Auxin acts like a morphogen at the root apex
      • Cells experiencing a low auxin level elongate and differentiate
        • Cells near the tip experience intermediate auxin levels and divide frequently
    • Ullagen differentiation and formation of root apical meristem are closely related to auxin signaling
      • During the first division of the embryo forming a two-cell stage, PIN 7 is localized to the top plasma membrane of the basal cell, inducing the transport of auxin from the adjacent basal cell to the top cell, in which auxin accumulates. At the spherical embryo stage, PIN 1 appears at the basal plasma membrane of the cell, meanwhile, PIN 7 turns to the basal plasma membrane of basal cells and PIN 4 begins to be expressed at the base of the embryo, thus inducing auxin transport from the embryonic cell to the radicagen and promoting radicagen development.
    • IAA is the major hormone that regulates apical cell division and differentiation
      • PIN 1 is a major mediator of IAA transport to quiescent centers and root crowns
      • PIN 2 is the major mediator of the reflux transport of IAA along the apical lateral root crown and epidermis to the basalmeristem zone
      • PIN 3 protein localized in root crown columnar cells mediate transport of auxin to the lateral root crown
      • PIN 4 is expressed in the quiescent center and surrounding cells and regulates the content of IAA in the quiescent center, makingthe quiescent center the highest point of IAA concentration